The present disclosure relates generally to toner compositions comprising a binder that comprises an amorphous polymer resin containing an unsaturated moiety, in which the surface is cross-linked to form a thin robust shell when surface treated with a peroxide. Additionally, exemplary embodiments relate to processes for forming such toner compositions. These toner compositions are particularly useful in xerographic or electrostatographic printing processes, and are described with particular reference thereto. However, exemplary embodiments are also useful in other applications.
Xerographic toners of a resin, a pigment, and a charge control agent are known. Toners useful for xerographic applications should exhibit certain performances related to storage stability, and particle size integrity; that is, the toner particles should remain intact and not agglomerate until fused on paper. The toner compositions also should not substantially agglomerate at temperatures below about 50° C. to about 55° C., because environmental conditions vary. The toner compositions should also display acceptable triboelectric properties that vary with the type of carrier or developer composition.
Another desirable property for xerographic toner compositions to possess is fusing property on paper. There is pressure to reduce the fusing or fixing temperatures of toners onto paper, for example, to fixing temperatures of from about 90° to about 120° C., to lower power consumption and to allow extended fuser system lifetimes. Non-contact fusers, which heat toner images on paper by radiant heat, usually are not in contact with the paper and the toner image. Contact fusers, on the other hand, are in contact with the paper and the toner image, and the toner compositions used with contact fusers should not substantially transfer onto the fuser roller.
Toner fixing performance can be characterized as a function of temperature. The maximum temperature at which the toner does-not adhere to the fuser roll is called the hot offset temperature (HOT). When the fuser temperature exceeds the toner's HOT, some of the molten toner adheres to the fuser roll during fixing and is transferred to subsequent substrates containing developed images. This transfer may result in blurred images. This undesirable phenomenon is called hot offset or cold offset depending on whether the temperature is below the fixing temperature of the paper (cold offset), or above the fixing temperature of the toner (hot offset).
The minimum fixing temperature (MFT) of the toner, which is the minimum temperature at which acceptable adhesion of the toner to the support medium occurs, should be as high as possible, but is always less than the toner composition's HOT. The MFT is determined, for example, by a crease test. The difference between MFT and HOT is called the fusing latitude of the toner, i.e., the temperature difference between the fixing temperature and the temperature at which the toner offsets onto the fuser.
Additionally, small-sized toner particles, such as those having average particle sizes of from about 3 to about 12 microns, such as from about 5 to about 7 microns, are desired, especially for use in high resolution xerographic engines. Small-sized toner particles can be economically prepared by chemical processes, which involve the direct conversion of emulsion-sized particles to toner composites by aggregation and coalescence, or by suspension, micro-suspension or micro-encapsulation processes.
Low temperature fixing toners comprised of semi-crystalline resins are known. For example, U.S. Pat. No. 5,166,026 discloses semi-crystalline copolymer resin toners, with melting points of from about 30° C. to about 100° C., and containing functional groups comprising hydroxy, carboxy, amino, amido, ammonium or halo, and pigment particles. Similarly, U.S. Pat. No. 4,952,477 discloses toner compositions of semi-crystalline polyolefin resin particles, with melting points of from about 50° C. to about 100° C., and containing functional groups comprising hydroxy, carboxy, amino, amido, ammonium or halo, and pigment particles. Although, some of these toners may provide low contact fixing temperatures of about 93.3° C. to about 107.2° C., the resins are derived from components with melting characteristics of about 30° C. to about 50° C., and are not believed to exhibit higher, more desirable melting characteristics, such as about 55° C. to about 60° C. The '026 and '477 patents are incorporated herein by reference in their entirety.
U.S. Pat. No. 4,990,424, the disclosure of which is incorporated herein by reference in its entirety, discloses toners comprised of a blend of resin particles containing styrene polymers or polyesters and components selected from the group consisting of semi-crystalline polyolefin and copolymers thereof with a melting point of from about 50° C. to about 100° C. Fusing temperatures of from about 250° F. to about 330° F. are reported.
Crystalline-based toners are disclosed in U.S. Pat. No. 4,254,207, the disclosure of which is incorporated herein by reference in its entirety. Low fixing toners comprised of cross-linked crystalline resin and amorphous polyester resin are illustrated in U.S. Pat. No. 5,147,747 and U.S. Pat. No. 5,057,392, the disclosures of which are incorporated herein by reference in its entirety, in which the toner powder is comprised, for example, of polymer particles of partially carboxylated crystalline polyester and partially carboxylated amorphous polyester that has been cross-linked together at elevated temperature with the aid of an epoxy resin and a cross-linking catalyst.
U.S. Pat. No. 5,916,725, the disclosure of which is incorporated herein by reference in its entirety, describes a process for preparing toner compositions comprising mixing an amine, an emulsion latex containing sulfonated-polyester resin, and a colorant dispersion, heating the resulting mixture, and optionally cooling.
U.S. Pat. No. 5,593,807, the disclosure of which is incorporated herein by reference in its entirety, provides a process for preparing toner compositions comprising (i) preparing an emulsion latex comprised of sodio-sulfonated polyester resin particles of from about 5 to about 500 nanometers in size diameter by heating the resin in water at a temperature of from about 65° C. to about 90° C.; (ii) preparing a pigment dispersion in water by dispersing in water from about 10 to about 25 weight percent of sodio-sulfonated polyester and from about 1 to about 5 weight percent of pigment; (iii) adding the pigment dispersion to the latex mixture with shearing, followed by the addition of an alkali halide in water until aggregation results as indicated, for example, by an increase in the latex viscosity of from about 2 centipoises to about 100 centipoises; (iv) heating the resulting mixture at a temperature of from about 45° C. to about 55° C. to cause further aggregation and enabling coalescence to form toner particles of from about 4 to about 9 microns in volume average diameter and with a geometric distribution of less than about 1.3; and optionally (v) cooling the product mixture to about 25° C. and washing and drying the product. The sulfonated polyesters disclosed in the '807 patent may be selected for use in embodiments.
Conventional low melt toner compositions, such as those described above, generally exhibit a ratio of amorphous resin to crystalline resin of 80 to 20, and meet the crease, gloss, latitude, and charging performance requirements of high-speed production printing. These toners also meet heat-cohesion requirements when less than 10% additives are present.
However, there remains a need for ultra-low melt toners with improved heat cohesion for non-additive heat-cohesion requirements and that still provide excellent image properties. There is thus also a need to provide a process for preparing such low-melt emulsion-aggregation toners that allows for controlled particle growth and controlled morphology or shape, and provides high yields.